SEC. III.] OF NATURAL PHILOSOPHY. 119 And the same way may it be demonstrated, that the body having its cen tripetal changed into a centrifugal force, will move in the conjugate hy perbola. LEMMA XIII. The latus rectum of a parabola belonging to any vertex is quadruple the distance of that vertex from the focus of thejigurc. This is demonstrated by the writers on the conic sections. LEMMA XIV. Tlie perpendicular, let fall from the focus of a parabola on its tangent, is a mean proportional between the. distances of the focus from the poini of contact, and from the principal vertex of the figure. For, let AP be the parabola, S its focus, A its principal vertex, P the point of contact, PO an ordinate to the principal diameter. PM the tangent meeting the principal diameter in M. and SN the perpendicular from the fo-~ M A s o cus on the tangent : join AN, and because of the equal lines MS and SP, MN and NP, MA and AC, the right lines AN, OP, will be parallel ; and thence the triangle SAN will be right-angled at A, and similar to the equal triangles SNM, SNP j therefore PS is to SN as SN to SA. Q.E.D. COR. 1. PS2 is to SN2 as PS to SA. COR. 2. And because SA is given, SN- will be as PS. COR. 3. And the concourse of any tangent PM, with the right line SN. drawn from the focus per] endicular on the tangent, falls in the right line AN that touches the parabola in the principal vertex. PROPOSITION XIII. PROBLEM VIII. If a body moves in the perimeter of a parabola ; it is required to find the. law of the centripetal force tending to the focus of that figure. Retaining the construction of the preceding Lemma, let P be the body in the perimeter of the parabola ; and from the place Q,, into which it is next to succeed, draw QH parallel IS!. and Q,T perpendicular to SP, as also Qv parallel to the tan gent, and mating the diame ter PG in v, and the distance —